Not Applicable.
Not Applicable.
1. Scope of Invention
This invention relates to the field of pressure switches. More specifically, the present invention relates to a circuit board integrated pressure switch that can be calibrated to a desired set point at the time of assembly or after assembly.
2. Description of the Related Art
Pressure switches are known to those skilled in art. Typically, these devices fall into two categories. In the first category are set-point pressure switches that actuate when a specified pressure is reached. The second category contains the more sophisticated pressure measuring switches that are capable of measuring the ambient pressure and reacting accordingly. Generally, the pressure switches in the first category are less complex and more cost efficient than those in the second category.
In low tire pressure warning systems, a typical mass-market application, the critical function of the warning system is to identify when the tire pressure falls below a specified value. Accordingly, using a pressure sensor with the ability to measure the tire pressure adds extra expense to the system.
FIG. 1 illustrates an exploded view of a conventional pressure switch. The conventional pressure switch includes a base member 11 having tabs 12 for securing the conventional pressure switch on a circuit board. Connected to the base member 11 is a metal enclosure 13. A hollow contact pin 14 passes through an opening in the base of the metal enclosure 13 and allows the pressure of and/or the gas within the pressure switch to be changed. A non-conductive end piece 15 serving as a diaphragm support is attached to the end of the contact pin 14 terminating within the metal enclosure 13. A thin metal diaphragm 17 covers the open end of the metal enclosure 13. A metal spring 16 biases the end piece 15 and the contact pin 14 against the diaphragm 17. The metal spring 16, in combination with the hollow contact pin 14, is selected to set the pressure at which the switch responds. A cover 18 having a small, central through opening 19 provides protection for the diaphragm 17.
An apparatus integrated onto an electronic circuit board for sensing when the ambient pressure reaches a specified value, or pressure switch, is shown and described. The pressure switch is low profile, reliable and has a set point that can be configured at, or after, the time of installation of the associated circuit board. Further, the pressure switch can contain other electrical components to conserve valuable circuit board real estate and allow the construction of self-contained electronic pressure switches.
The pressure switch is an absolute pressure switch including a fluid-tight sealed enclosure having one surface that acts as a diaphragm. At least one other surface of the pressure switch is formed from a circuit board. The internal volume of the pressure switch is typically small and is maintained at the atmospheric pressure where and when the pressure switch is sealed. Variations in the ambient air pressure relative to the internal pressure force the diaphragm to move thereby making or breaking contact with an electrical conductor that completes the switch. Finally, the internal volume can be filled with either atmospheric gases or with an inert gas selected to prevent oxidation or to change the set point characteristics.
In one embodiment, the pressure switch includes a number of rings stacked on the surface of the circuit board to create an open-ended enclosure having the desired volume. In the illustrated embodiment, the enclosure is a cylinder with the circuit board forming the base of the cylinder. In another embodiment, the enclosure of the pressure switch is formed using the thickness of the circuit board. In this embodiment, the circuit board defines a large diameter plated through opening. A trace is etched around the edges of the through opening to cooperate with the solder paste to provide the sealing function. A base plate is secured to one side of the circuit board. The base plate defines a contact opening adapted to receive a conductive member. The end of the conductive member is positioned at a distance relative to or in contact with the diaphragm while the pressure switch is under pressure to define the appropriate set point. The conductive member is secured in a fluid-manner, e.g., soldered into place.
The diaphragm is secured to the open end of the enclosure in a fluid-tight manner, completing the sealed enclosure. The diaphragm is constructed from an electrically conductive, flexible material. The diaphragm includes a series of concentric grooves to provide extra flexibility such that the diaphragm can move without deformation. Both the top and bottom surfaces of the diaphragm are provided with a set of concentric grooves. Within each set, the concentric grooves are equally spaced; however, the top set and the bottom set are offset from one another. The present inventors have found that this arrangement provides increased flexibility and responsiveness in the diaphragm. The diaphragm is designed to exhibit flexibility and allow movement in response to a pressure differential between the ambient pressure and the internal air pressure of the pressure switch.
The circuit board defines a through opening providing access to the interior of the enclosure. The through opening is dimensioned to receive a conductive member that serves to define the set point of the pressure switch. The conductive member is secured within the through opening in a fluid-tight manner, e.g., soldered into place. To assign a set point, the pressure switch is placed under pressure. Typically this is achieved by applying a calibrated pressure on the diaphragm to induce movement. The conductive member is installed through the through opening in the bottom of the volume until it is brought in contact with the distended bottom surface of the diaphragm and soldered into place to create a fluid-tight seal. When pressure is released, the diaphragm returns to an unflexed position thereby placing the pressure switch into its normal state. Depending upon the application, the pressure switch can be configured to react when the ambient pressure either goes above or below a selected set-point.